1. Field of the Invention
The present invention relates to a fuel cell system provided with a fuel cell and an electric storage device.
2. Description of Related Art
In recent years, there have been developed various types of fuel cell systems provided with a fuel cell and an electric storage device. In the fuel cell system provided with a fuel cell and an electric storage device, the electric storage device needs to be charged so as not to become empty.
When the electric storage device is charged, it is usually necessary to stop the operation of an external load of the fuel cell system. The problem here is that the capacity of the electric storage device limits the duration for which the fuel cell system can be continuously used.
To overcome this problem, Japanese Patent Application Laid-Open No. 2004-71260 discloses a fuel cell apparatus built as a system in which a fuel cell and a rechargeable battery as an electric storage device are provided in parallel. In this system, the rechargeable battery is charged when the output voltage of the fuel cell is higher than the output voltage of the rechargeable battery. This permits the rechargeable battery to be charged without stopping the operation of the external load. However, in the fuel cell apparatus disclosed in Japanese Patent Application Laid-Open No. 2004-71260, the rechargeable battery is charged even when there is no surplus electric power (=the maximum electric power of the fuel cell minus the load electric power). This may result in insufficient electric power supply for the load.